Conventionally, when vulcanizing large tires such as large OR tires or the like, hot water vulcanization is used in which hot water is injected into the bladder to fill it and vulcanize the green tire. This is because the temperature setting range of hot water vulcanization is large compared with steam vulcanization, and the pressure can be increased. Also, the energy density per unit volume of hot water is high, so it is suited to vulcanization of large tires whose wall thickness is comparatively large.
However, in the case of hot water vulcanization the hot water that contacts the internal walls of the bladder loses heat, and the temperature drops. As a result the density of the water increases, and the water moves downward as a cold water mass and accumulates at a height that is at the same temperature. Therefore a vertical temperature difference is produced within the bladder, and a temperature difference is also produced in the outer surface of the bladder in contact with the green tire. For example, the maximum temperature difference in the vulcanization temperature can be about 50° C.
In the case of steam vulcanization, saturated water vapor that heats the inner wall of the bladder immediately condenses, and flows down the inner wall. If the consumed steam is replenished the same thing is repeated when the saturated water vapor continuously contacts the inner wall of the bladder, so the inner wall of the bladder is maintained at the constant temperature of the saturated water vapor. Therefore, a uniform temperature can be maintained except in the drain where the condensed water accumulates.
If the vertical temperature difference in the vulcanization bladder as described above is excessive, the quality of the vulcanized tire will be variable, so it is important to make the temperature difference of the bladder as small as possible. The problem of the temperature difference in the bladder described above also occurs for gas vulcanization using steam as the heating medium and nitrogen gas or the like as the pressurizing medium
Although not for hot water vulcanization, various methods and devices have been proposed to reduce the temperature difference when vulcanizing a green tire by agitating the heating medium within the bladder (for example, see Japanese Unexamined Patent Application Publication No. S56-28843 and Japanese Unexamined Patent Application Publication No. H06-238670). However, these conventional methods and devices were unable to sufficiently reduce the temperature difference of the bladder.